Here is where you add content to the face of the pass. Each field requires a unique name that will be used by the API when updating the pass. Each pass type has a prescribed layout and a maximum number of fields. Apple Wallet will automatically shrink long text values and very long text values may be cut short.
Use the dynamic option if the label or data value will be different on each pass. If you add a change message, an alert will appear on the user's lock-screen or notification centre whenever the pass is updated with new data.
Each pass may contain a barcode that can be scanned by the pass issuer to identify a pass or to trigger some sort of action (E.g. update a field on the pass and push an update). Apple Wallet supports 4 types of barcode. Which barcode is most appropriate will depend upon the amount of information you wish to encode, the capability of your scanning technology and the design of your pass.
Square barcodes (Aztec and QR Code) work better for large messages but reduce the amount of other information the pass can display for certain pass types. Code 128 Barcodes require a fallback fro devices running iOS 8 or earlier.